The novel powder compositions of this invention are improvements over those disclosed and claimed in Bouchard U.S. Pat. No. 3,583,931, issued June 8, 1971. That patent teaches the benefits of bismuth and ruthenium and/or iridium in polynary oxides having pyrochlore-related crystal structure for electrical resistor applications. U.S. Pat. No. 3,553,109 to Lewis C. Hoffman teaches resistor compositions comprising such polynary oxides (and related polynary oxides) plus inorganic binder and finely-divided noble metals. Resistor compositions according to these teachings have enjoyed considerable commercial success because of the excellent control they offer in providing a range of resistors with reproducible values of resistivity, little affected by temperature or humidity in use, and readily printed and fired on dielectric supports.
A primary means of establishing the resistivity of a fired resistor according to the teaching of Hoffman is to adjust the relative proportions of polynary oxide, noble metal, and inorganic binder in the composition. In general, the unusual properties of the polynary oxide are adjusted toward higher resistivities by increasing the proportion of binder and adjusted toward lower resistivities by increasing the proportion of finely divided noble metal. However, increasingly large proportions of noble metal to polynary oxide also produce an increase in TCR (temperature coefficient of resistance) and obviate many of the advantages which have led to the gradual replacement of noble metal/glass compositions (such as the palladium/silver/glass compositions of D'Andrea U.S. Pat. No. 2,924,540) by the more sophisticated polynary oxide containing compositions.
Chemical substitution in the polynary oxide itself has been investigated as a method for adjusting electrical properties. Thus the patents cited above contemplate, e.g., the substitution ot yttrium, thallium, indium, cadmium, lead or the rare earth metals of atomic number 57-71 inclusive for some of the bismuth in Bi.sub.2 Ru.sub.2 O.sub.7 and Bi.sub.2 Ir.sub.2 O.sub.7 ; and platinum, titanium, tin, chromium, rhodium, rhenium, zirconium, antimony or germanium for some of the ruthenium or iridium. There is a need, however, for compositions capable of producing resistivities that are substantially lower than those obtained with Bi.sub.2 (Ru,Ir).sub.2 O.sub.7, while maintaining the desirably flat temperature response. Such low resistivities are often below 10 ohms/square, preferably 1-5 ohms/square.
U.S. Pat. No. 3,324,049 teaches cermet resistance materials of glass and one or more oxides or Ir, Os and Ru, the glass containing "CuO dissolved in the glass and forming a structural part of the glass." Polynary oxides of pyrochlore-related crystal structures are neither disclosed nor suggested. In fact, not even one noble metal oxide is specifically disclosed. The CuO in the glass does not lower resistivity of the claimed materials, but to the contrary (at column 3, lines 45-48) it is disclosed that when an oxide of copper is added to the raw glass materials prior to fritting or to prefritted glass, there is substantially no change in ohmic resistance of the resultant resistor. Hence there is no reason to expect that CuO, or Cu metal, would lower resistivity of resistor compositions.